Linear RF ion trap with high mass resolution

Abstract

In a linear ion trap in which an essentially quadrupole RF electrical field is generated between at least four rod-shaped electrodes, ions may be mass-selectively ejected orthogonally to the axis. An aspect of the invention comprises compensating for field irregularities along the axis of a linear ion trap, which result, at different ejection locations, in the ejection of ions of the same masses at slightly different times, by of measuring the ions that are ejected at the different ejection locations using a number of separate detectors, and correcting, after a mass calibration of each of the mass spectra, the time shifts of the various location-dependent mass spectra during their addition to a combined spectrum.

1. A linear ion trap for a mass spectrometer, with radial ejection of the ions, comprising:
 n ion detectors for measuring the currents of the ejected ions, with n greater than one, and the n ion detectors are located at an exterior side and along a longitudinal axis of the linear ion trap, wherein each ion detector provides a location-specific ion signal. 
 
   
   
     2. The linear ion trap of  claim 1 , wherein the n ion detectors are followed by n amplifiers and n analog-to-digital converters, which generate n series of values from the currents in the n ion detectors; the n series of values represent n location-specific mass spectra. 
   
   
     3. The linear ion trap of  claim 2 , wherein a computing unit is incorporated, in which at least some of the n series of values are added value by value, with offsets considering time shifts between the location-specific mass spectra as measured by prior calibration, to form a cumulative series of values representing the total mass spectrum. 
   
   
     4. The linear ion trap of  claim 3 , wherein the computing unit incorporates a memory for calibration constants, with which the time offsets are controlled during addition. 
   
   
     5. The linear ion trap of  claim 3 , wherein a PC, an FPGA or a signal processor is used as the computing unit that adds the series of values to create the cumulative series of values. 
   
   
     6. The linear ion trap of  claim 5 , wherein a FIFO register is present for each series of values, and is used to buffer the values for the addition with offsets. 
   
   
     7. The linear ion trap of  claim 1 , wherein the n ion detectors comprise a multichannel plate with n electron collectors collecting the electrons emerging from the multichannel plate. 
   
   
     8. The linear ion trap of  claim 1 , comprising electronic means for generating an exciting field inside the linear ion trap to resonantly eject the ions. 
   
   
     9. The linear ion trap of  claim 8 , comprising at least four pole rods, the pole rods being shaped or displaced to superimpose multipole fields on an essentially quadrupolar field inside the ion trap, thus generating nonlinear resonance phenomena inside the ion trap. 
   
   
     10. The linear ion trap of  claim 9 , wherein the frequency of the exciting field is chosen such that the resonant ejection of ions is enhanced by one of the nonlinear resonances. 
   
   
     11. The linear ion trap of  claim 10 , wherein, by a mixture of higher multipole fields superimposed on the quadrupole field, the radial ejection of ions is directed to one side only, and wherein the ion detectors are arranged at one side of the ion trap only. 
   
   
     12. The linear ion trap of  claim 11 , wherein the mixture of multipole fields superimposed on the quadrupole field essentially comprise a hexapole and an octopole field. 
   
   
     13. The linear ion trap of  claim 1 , wherein adjustable delay lines are incorporated, in which the analog currents from the n ion detectors can be given an adjustable mass-proportional delay, before they are added and digitized. 
   
   
     14. A linear ion trap, comprising:
 a quadrupole ion trap that includes four pole rods that form a chamber for containing ions, where a first of the four pole rods includes a through slit from an interior side of the first of the four rods to an exterior side of the first of four pole rods, such that ions may pass from the interior side through the slit to the exterior side; and 
 a plurality of ion detectors located lengthwise adjacent to the slit on the exterior side of the first rod, where each ion detector provides a uniquely associated detected ion signal. 
 
   
   
     15. The linear ion trap of  claim 14 , where the plurality of ion detectors are configured and arranged in a straight line parallel to a longitudinal axis of the slit. 
   
   
     16. The linear ion trap of  claim 15 , where the plurality of ion detectors includes at least eight detectors. 
   
   
     17. The linear ion trap of  claim 15 , where the width of the slit decreases from the exterior surface to the interior surface. 
   
   
     18. The linear ion trap of  claim 17 , where a multichannel plate is located between the slit and the plurality of detectors. 
   
   
     19. The linear ion trap of  claim 18 , comprising a plurality of analog-to-digital converters that each receives a uniquely associated one of the detected ion signals. 
   
   
     20. A linear RF ion trap, comprising:
 an ion trap that includes four pole rods that form a chamber for containing ions, where a first of the four pole rods includes a lengthwise through slit from an interior side of the first of the four rods to an exterior side of the first of four pole rods, such that ions may pass from the interior side through the slit to the exterior side; 
 a plurality of ion detectors located lengthwise adjacent to the slit on the exterior side of the first rod, where each ion detector provides a uniquely associated detected ion signal; and 
 a controller that adds the detected ion signals with offsets considering time shifts between the location-specific mass spectra as measured by prior calibration, to form a cumulative series of values representing the total mass spectrum. 
 
   
   
     21. A method of measuring a mass spectrum with a linear ion trap having a longitudinal axis, comprising: injecting ions into the linear ion trap; trapping injected ions in the linear ion trap; mass selectively ejecting the ions from the linear ion trap in a radial direction with respect to the longitudinal axis; measuring location-specific mass spectra with a plurality of ion detectors; and combining the location-specific mass spectra data provided by the plurality of detectors into a sum spectrum by correcting mass shifts of the location specific mass spectra. 
   
   
     22. The method of  claim 21 , where the measuring location-specific mass spectra with a plurality of ion detectors comprises measuring the location-specific mass spectra along a first exterior side of the linear ion trap and measuring the location-specific mass spectra along a second exterior side of the linear ion trap.